The increasing demand in energy storage has stimulated a significant interest in lithium-battery research. Most commercially available lithium-ion batteries include graphite based anodes, wherein the theoretical capacity of graphite is 372 mAh/g. In order to increase the energy density of the lithium batteries, higher capacity anodes are required. Silicon has attracted considerable attention in the field of Li-batteries due to its theoretical capacity of 4200 mAh/g, which is an order of magnitude greater than that of graphite. Additionally, Si exhibits a low de-lithiation potential against Li/Li+, such that high battery voltages can be reached. Furthermore, silicon is a low-cost and environmentally-friendly material, and is the second most abundant material on Earth.
The main disadvantage of high-capacity anode materials, such as Si, is their particularly large volume expansion and contraction during Li insertion/de-insertion, followed by cracking and pulverization of the anode material. For instance, silicon exhibits up to about 320% volume expansion upon complete alloying with lithium, thus inducing a rapid degradation of Si-based anodes. One plausible way to deal with the detrimental pulverization is to reduce the size, and/or thickness of the anode down to the nanoscale. Several approaches have been reported, including the use of nanospheres, nanotubes, nanowire arrays and porous structures (Y. Yao, M. T. McDowell, I. Ryu, H. Wu, N. A. Liu, L. B. Hu, W. D. Nix, Y. Cui Nano Lett., 11 (2011), pp. 2949B. Hu; H. Ma, F. Y. Cheng, J. Chen, J. Z. Zhao, C. S. Li, Z. L. Tao, J. Liang Adv. Mater., 19 (2007), p. 4067; T. Song, J. L. Xia, J. H. Lee, D. H. Lee, M. S. Kwon, J. M. Choi, J. Wu, S. K. Doo, H. Chang, W. Il Park, D. S. Zang, H. Kim, Y. G. Huang, K. C. Hwang, J. A. Rogers, U. Paik Nano Lett., 10 (2010), pp. 1710; M. H. Park, M. G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, Y. Cui, J. Cho, Nano Lett., 9 (2009), pp. 3844).
Si nanostructures have the advantage of a shorter diffusion distance for lithium species, which can improve the power performance of the battery. It has been shown that the high surface-to-volume ratio of nanoparticles helps to better withstand stress, and substantially limit the cracking extent. The existence of a strong particle size-dependent fracture behavior of Si nanoparticles during the first lithiation cycle was shown experimentally; that is, there exists a critical particle size of ˜150 nm below which cracking does not occur, and above which surface cracking and particle fracture is observed. Silicon nanowires (SiNWs) provide a highly porous medium, which allows easy expansion of silicon during lithium insertion.
There are two main approaches for the preparation of silicon nanowires: growth methods and etching methods. The vapor-liquid-solid (VLS) mechanism, discovered about 50 years ago by Wagner and Ellis is the most popular of the growth methods (R. S. Wagner and W. C. Ellis, Appl. Phys. Lett., 1964, 4, 89-91). VLS growth is usually performed in a chemical-vapor-deposition (CVD) reactor, by decomposition of silicon-bearing gases, like silane (SiH4) or silicon tetrachloride (SiCl4), over a temperature range of about 300-1000° C., depending on the gas precursor and the type of metal catalysts employed. Silicon NWs can be grown on different types of metal catalysts, like Au, Cu, Ag, In, Ga, Zn and others.
SiNWs for rechargeable Li battery applications, grown on the surface of a substrate, for example on stainless steel, generally have a “forest” structure (C. K. Chan, H. L. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31-35; B. Laïk, L. Eude, J.-P. Pereira-Ramos, C. S. Cojocaru, D. Pribat and E. Rouvière, Electrochim. Acta, 2008, 53, 5528-5532). The main drawbacks of this approach are: low surface capacity (typically less than 1 mAh/cm2), very high irreversible capacity (about 30%), which is required for the formation of the SEI (solid electrolyte interphase), and insufficient current efficiency (typically 95 to 99.5%). In contrast, in the state-of art lithium-ion-battery technology, employing graphite-based anodes, the irreversible capacity is about 10% or less, the surface capacity is about 3-4 mAh/cm2, and the current efficiency is over 99.9%. Furthermore, most publications on SiNWs-based anodes demonstrate a single desired property (low Qir, high surface capacity, high electrode capacity (mAh/gSi), high current efficiency or high cycle number) but not all of said properties are achieved for the same electrode. In most cases, good performances were demonstrated for very low and impractical surface capacity only. Furthermore, it was shown that SiNW forest agglomerates to a thick, solid mass of Si near the substrate, during the lithiation and de-lithiation processes, leading to the delamination of the SiNWs from the substrate (A. Kohandehghan, P. Kalisvaart, M. Kupsta, B. Zahiri, B. Shalchi Amirkhiz, Zh. Li, E. L. Memarzadeh, L. A. Benderskyc and D. Mitlin, J. Mater. Chem. A, 2013, 1, 1600-1612).
U.S. Pat. No. 8,637,185 is directed to conductive substrates having open structures and fractional void volumes of at least about 25% or, more specifically, or at least about 50% for use in lithium ion batteries. Nanostructured active materials are deposited over such substrates to form battery electrodes. In specific embodiments, a nanoscale layer of silicon is deposited over a metallic mesh to form a negative electrode. In another embodiment, a conductive substrate is a perforated sheet with multiple openings, such that a nanostructured active material is deposited into the openings but not on the external surfaces of the sheet.
Silicon nanowires or whiskers supported on carbonaceous materials or on silicon substrates, for use as Li-ion battery anode have also been reported.
U.S. Pat. No. 8,791,449 is directed to a process for etching a silicon-containing substrate to form nanowire arrays, which can be used for manufacturing an anode material for lithium ion batteries comprising nanostructured silicon.
US Patent Application No. 2011/0117436 is directed to carbon nanofibers having a surface and including at least one crystalline whisker extending from the surface of the carbon nanofiber, and to battery anode compositions that can be formed from a plurality of carbon nanofibers each including a plurality of crystalline whiskers.
International Patent Application No. 2013/052456 discloses nanostructured materials including silicon-based nanostructures such as silicon nanowires and coated silicon nanowires, nanostructures disposed on substrates comprising active materials or current collectors such as silicon nanowires disposed on graphite particles or copper electrode plates, and lithium-ion battery anode composites comprising high-capacity active material nanostructures formed on a porous copper and/or graphite powder substrate.
A recent study used Si nanowires grown on a conducting carbon-fiber support to provide a robust model battery system that can be studied by 7Li in situ NMR spectroscopy (K. Ogata, E. Salager, C. J. Kerr, A. E. Fraser, C. Ducati, A. J. Morris, S. Hofmann & C. P. Grey, Nature Communications 5, 3217 (2014)).
Additional problem related to depositing or growing high loading SiNWs on a substrate surface or applying SiNWs to a conductive substrate, in order to incorporate said SiNWs into an anode structure, is a poor electric contact between the nanowires and the substrate and lack of direct electron conduction path to the substrate. Furthermore, in all lithium batteries the anode is covered by a thin solid electrolyte interphase (SEI), which is formed during the first charging cycle. Ideally, this SEI is permeable to lithium ions, while being an electronic insulator, thus preventing or slowing down further electrolyte decomposition during the cycles that follow. However, in the case of the silicon-based anodes, “breathing” of the anode material during insertion/de-insertion of lithium causes cracks, exposing the bare silicon surface to the electrolyte, and this is followed by the creation of a fresh SEI, thus losing battery capacity and increasing battery impedance (H. Wu, G. Chan, J. W. Choi, I. Ryu, Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu and Y. Cui, Nat. Nanotechnol., 2012, 7, 310-315). Reduction in silicon nanowire diameter with number of cycles due to SEI formation was also reported, while significantly greater Si loss was near the nanowire base, which was in contact with the current collector (J.-H. Cho and S. T. Picraux, Nano Lett. 2014, 14, 3088-3095). Additionally, the low electrical conductivity of Si sometimes requires the use of conductive additives in the anode film.
There remains an unmet need for the improved silicon nanostructures-based anodes, in particular for Li-ion batteries, which would meet the requirements of said batteries for portable and electric-vehicle applications. The silicon nanostructures-based anodes should be capable of providing high capacity, low irreversible capacity, high current efficiency and a stable cycle life.